This invention relates to fabrication of bipolar junction transistors (BJTs) particularly adapted for fusing capability. Specifically, the invention relates to a BJT vertical fuse having a low dose emitter in a thin epitaxial layer. The invention also relates to a fabrication of BJT vertical fuse when processing methods interpose polycrystalline silicon (polysilicon) between a fuse contact and an emitter.
Douglas Peltzer, in U.S. Pat. No. 3,648,125, provides an example of a process for an oxide isolated BJT integrated circuit. The Peltzer patent is expressly incorporated by reference for all purposes. Many integrated circuits include some type of fusing structure. The fuse structure provides an ability to permanently switch or alter some aspect of an integrated circuit's function after fabrication. These fuse structures may either be lateral or vertical. Lateral fuses extend horizontally across a die's surface, thereby taking up valuable area. Vertical fuses are relatively space efficient in that the fusing areas overlie one another.
A vertical fuse may be a modified BJT. The typical BJT includes a collector region disposed in a substrate. A base region overlies the collector region and an emitter region overlies the base region. Polysilicon, aluminum, or aluminum alloy contacts, interconnect various structures of the integrated circuit. Special procedures used during processing ensure that each collector, base and emitter region has a low resistance path to the surface, allowing the metal contact to communicate with each of them. Vertical fuses are floating base BJTs. A floating base BJT is a BJT that dispenses with contact to the base region.
Programming a vertical fuse, that is "blowing the fuse", results from reverse biasing the floating base BJT. The reverse bias current generates heat in the vertical fuse. Although not thoroughly understood, one possible explanation of the fusing action is that the generated heat is sufficient to raise the temperature of the emitter to about 550.degree. C. 550.degree. C. is the eutectic point for aluminum and silicon. At the eutectic point, silicon from the emitter and aluminum from the contact form a liquid which flows into a void created by the solution of the silicon into the aluminum. After programming, the void extends completely through the emitter producing a low ohmic contact to the base region from the aluminum contact. The low ohmic contact shorts the emitter and effectively provides a diode structure after programming. This change of the vertical fuse from a floating base transistor to a diode is detectable by external circuitry. Providing an array of the fuses and selectively programming particular ones of the fuses can produce a programmable read only memory (PROM) or a programmable array logic (PAL) device, for example. Assignment of an unprogrammed fuse with a value of "0" and a programmed fuse with a value of "1" provides for the PROM or the PAL device. Thus, the vertical fuse's greater packing density allows a use of large arrays with small die areas, contributing to the vertical fuse's use in PROMs and PAL devices.
Two problems which develop by use of conventional BJTs in production of dense fuse arrays include overblowing and crosstalk. Overblowing occurs when the ohmic contact not only extends through the emitter region, but also extends through the base region. This overblow produces an irreversible Schottky diode which functions differently from the programmed fuse. Formation of a Schottky diode results in a leaky device which has different forward characteristics that tend to be unrepeatable. An access of the Schottky diode may provide too much or too little current, and may not function in the device.
Crosstalk is the term coined for two fuses interfering with one another. For example, programming a first fuse of an array may prevent programming of a second adjacent fuse if crosstalk is excessive.
A conventional BJT's design minimizes switching speed and maximizes transistor gain (.beta.). This design includes thin basewidths which provide in turn, relatively high fuse series resistance (R.sub.s), high .beta., low open-base breakdown voltage between the collector and the emitter (BV.sub.ceo) and low open-base breakdown voltage between the emitter and the collector BV.sub.eco. Conventional BJTs typically have .beta.'s greater than about 100-150, with BV.sub.ceo of about 6-8 volts and BV.sub.eco of about 2-2.5 volts These values are, unfortunately, unacceptable for optimal fusing action. Fuses desirably have a BV.sub.eco of greater than about 8 volts, with 10-12 volts preferred, and a BV.sub.eco between about 3.0 and 3.6 volts. Beta's less than about 10, and more preferably less than about 5, are desirable.
Thus, conventional BJT's do not have optimal performance parameters to promote their programmability when configured as a fuse. Providing for acceptable floating base BJTs encounters further difficulties upon consideration of the processing environment for these devices. For integrated circuits, processing forms hundreds of thousands to millions of transistors at one time. Usually, only a relatively small number of these transistors are to become fuses. The fuses must therefore, be made alongside conventional BJTs with minimal processing impact upon the conventional BJTs. That is, changes made to the processing to provide improved fuses must not degrade the performance of the non-fuse BJTs.
An additional problem with manufacture of vertical fuses results from use of polysilicon as an interconnecting medium. U.S. Pat. No. 4,764,480 issued Aug. 16, 1988 to Vora illustrates use of polysilicon as an interconnecting medium to provide contacts to desired active areas of an integrated circuit. The Vora patent is hereby expressly incorporated by reference for all purposes. A BJT made with a process using polysilicon typically has an epitaxial layer grown over a doped substrate. The doped substrate provides the collector region and an implant or diffusion into the epitaxial layer provides the base region. A polysilicon layer grown over the epitaxial layer receives dopants, some of which are driven into the underlying epitaxial layer to form an emitter and a collector sink. The doped regions of the polysilicon form ohmic contacts to which one or more metal layers provide necessary contacts to the active structures of the BJT. These polysilicon ohmic contacts are called emitter contacts if they overly emitter region in the epitaxial layer. The polysilicon layer separates the aluminum-containing metal contact and the emitter formed in the epitaxial layer. This separation makes conventional fusing action difficult to initiate. Implementation is especially difficult when it is necessary to preserve fast transistor characteristics for non-fuse BJTs proximate the vertical fuse BJTs and to minimize the impact on the processing sequences to develop the integrated circuit structures.
Fusing is difficult after use of polysilicon as an interconnecting medium because the temperature at which polysilicon becomes molten is about 1415.degree. C. Thus, melting the layer of polysilicon during programming would generate so much heat that damage to the device results.
From the above it is seen that improved vertical fuses and a method of fabrication thereof are desired.